In the production of components of Si.sub.3 N.sub.4 ceramic, powders are prepared in a first step and are then converted by suitable molding techniques into so-called green compacts which are sintered to the final ceramic in a further step. In addition to compression molding and injection molding, slip casting has proved more and more to be a suitable molding process in recent years. Slip casting has the advantage that it can be carried out using a suspension in which the powder particles are present in completely deagglomerated form, whereas compression molding in particular has to be carried out with finely divided, dry powders which inevitably form agglomerates. Even after sintering, however, agglomerates in the powder can still have adverse effects by reducing the strength and reliability of the component (J. P. Torre, Y. Bigay, Ceram. Eng. Sci. Proc. 7 (1986), 893-899).
The most important step in slip casting is the preparation of a stable, deflocculated dispersion of the Si.sub.3 N.sub.4 powder particles in the solvent, i.e. generally in water. To eliminate the effect of small, but uncontrolled ion concentrations, a certain ion strength is generally established by the addition of defined salts. The process is generally carried out in dilute (0.001M) KNO.sub.3 solution. However, unwanted flocculation often occurs in these suspensions. This is prevented by addition of organic dispersion aids.
However, it is known in the case of Si.sub.3 N.sub.4 that both the powder and the green compact should be free from carbon (G. Ziegler, J. Heinrich, G. Wotting, J. Mater. Sci. 22 (1987), 3041-3086). Accordingly, it is a disadvantage to stabilize Si.sub.3 N.sub.4 suspensions by addition of organic dispersion aids because residues of carbon from the dispersion aid can remain behind in the compact after molding.
After molding by slip casting, the finished ceramic component is produced from the green compact by sintering. High sintering densities presuppose a certain content of oxygen in the Si.sub.3 N.sub.4 powder. On the other hand, however, the oxygen content should not be too high because high oxygen contents reduce the strength values at high temperatures (G. Ziegler, J. Heinrich, G. Wotting, J. Mater. Sci. 22 (1987), 3041-3086). A total oxygen content of 1.5% by weight is regarded as optimal. If the oxygen content exceeds 1.8% by weight, the high-temperature properties can be expected to deteriorate.
Now, the object of the present invention is to provide Si.sub.3 N.sub.4 powders which have a total oxygen content of less than 1.8% by weight and good dispersion properties.